The present invention relates generally to a wastewater treatment process and, more particularly, to a process for removing microbial contaminants, including pathogenic microorganisms, contained in wastewater.
Pathogenic microorganisms are disease-causing agents that can contaminate domestic and recreational water supplies through the discharge of poorly treated industrial and municipal wastewater. Pathogenic microorganisms transmitted in this manner can cause major health problems to local communities and is often a causative factor in outbreaks of dysentery, cholera, typhoid, gastroenteritis, and other diseases. To minimize the chances of such outbreaks occurring, effluent standards are imposed on facilities which discharge treated wastewater into the environment.
Most pathogenic microorganisms can be classified as protozoan, bacteria, and viruses. The viruses of particular concern to wastewater treatment are water-borne polio viruses, including rota viruses. Bacteria comprise the largest group of pathogenic microorganisms and include Salmonella sp., Shigella sp., Escherichia coli, and a broad variety of others generally known. The most common bacteriological diseases include shigellosis, which causes dysentery, food poisoning, and cholera. Protozoan pathogens include Giardia sp. which causes giardiasis, one of the most prevalent water-borne diseases in the United States, and Cryptosporidium sp., which causes dysentery.
The treatment processes employed in wastewater treatment may be categorized as primary, secondary, and tertiary. In typical primary treatment, larger suspended solids are removed from raw wastewater using mechanical means and/or by gravity settling. In secondary treatment, biological treatment processes may be employed to metabolize organic matter to solid material and byproducts, followed by the removal of solid matter, usually by gravity settling. Secondary treatment may also include biological nutrient removal. Treatment beyond secondary treatment is commonly referred to as tertiary treatment. More recent tertiary treatment processes have employed physical-chemical or biological processes followed by chemical precipitation to lower the concentration of pathogenic microorganisms and other fine suspended solid particles in secondary-treated wastewater.
Filtration techniques are also used in some tertiary treatment processes. More specifically, granular media type filters such as deep-bed sand filters are used to strain out fine particles from the wastewater stream prior to disinfection. The degree to which these filters can separate pathogenic microorganisms from a wastewater stream varies with respect to the type of pathogenic microorganisms in the stream and, more particularly, to the size of the microorganisms. While most bacteria are about 10 microns or less in size, most protozoans are less than 5 microns and most viruses are less than 0.1 microns. The use of granular media type filters has proven more effective in removing some larger particles from wastewater streams than in removing small particles such as viruses. Deep bed multi-media filters have been employed to remove up to 30% to 40% of viruses and bacteria from a given volume of wastewater.
Despite prior treatment efforts, outbreaks of diseases continue to be linked to water-borne pathogenic microorganisms contaminating domestic water supplies and recreational water supplies. These occurrences have generated increasing pressure on responsible authorities to implement effluent standards which further reduce the concentrations of microbial contaminants, including pathogenic microorganisms, that can be discharged in treated industrial and municipal wastewater. Various treatment techniques are available for improving the degree to which microbial contaminants can be removed from a wastewater stream. These treatment techniques vary in cost, complexity, as well as in effectiveness.
Filtration techniques, particularly the use of sand filters and other granular-media type filters, may be advantageous as a tertiary treatment process because these techniques typically involve a simple operation and can be very cost effective compared to most chemical and biological processes. However, there is a general perception in the wastewater treatment industry that the effectiveness of filtration techniques to remove microbial contaminants, including pathogenic microorganisms, from wastewater cannot be substantially improved beyond the removal rate or degree currently achievable with sand filters and other granular media type filters.